This invention relates to a system and method for suppressing noise due to magnetic fields emanating from outside of an area in which biomagnetic fields are being measured.
Magnetic fields from biological sources such as the brain or heart are about 7 to 8 orders of magnitude smaller than the background geomagnetic field. Measurement of such small magnetic fields is possible using a Superconducting Quantum Interference Device (SQUID) and associated electronics; however, the smaller biological signals are usually swamped by the background terrestrial magnetic noise unless some attention is paid to suppressing this noise. There are two methods for doing this. The first method is to shield the desired source and sensor from the interference field using closed chambers of highly conducting and/or permeable materials which either exclude the magnetic interference or "channel" it around the measurement region. Good results are obtained by this method and commercial biomagnetometry systems employ this technique. However, the construction of such shielded chambers is costly, both in terms of material and real estate.
The second method is to suppress the interference fields by constructing a sensor which is much less sensitive to the interfering magnetic fields than it is to the desired magnetic fields. Such sensors are called gradiometers, since they are designed to be sensitive to some higher order gradient (or spatial derivative) of the magnetic field. This scheme takes advantage of the fact that the sources of magnetic interference are much further from the sensors than the biomagnetic sources, and, consequently, their magnetic gradients are smaller (at least as measured by the "finite difference" arrangement of the gradiometer coils). It can be shown that if dipole-like sources of interference are more than 150 ft. away, then a second-order gradiometer will provide as much attenuation of that interference as a clinical quality magnetically shielded room, but at a mere fraction of the cost.
Unfortunately, one can not build a perfect gradiometer, since they are constructed using a particular mechanical arrangement of wound coils of wire. Alignment is critical to obtaining ideal gradiometer performance. For example, a second-order gradiometer "as built" will probably respond to first and zeroth order magnetic fields as well; this is referred to as the "common mode response". Superconducting trim tabs are mechanically positioned around the gradiometer in order to minimize the common mode response, but these adjustments are tedious and prone to change with time. Another problem with mechanical trim adjustment is that it rapidly gets out of hand when there are a large number (&gt;20) or gradiometers, such as would be found in the next generation biomagnetometers.
There is therefore a need to improve upon the conventional gradiometer to achieve higher order gradiometer responses for even better suppression of distant magnetic interference without the attendant difficulties of balancing and trimming.